The spread of bovine TB into other species could strengthen the case for a badger cull, agricultural experts have predicted.

Latest figures from the Department of the Environment, Food and Rural Affairs (Defra) show only 28 alpacas, along with a handful of pigs, cats and dogs, have died after being infected with the disease in the first six months of this year.

But those in the industry say the statistics are misleading, due to the way the data is collected.

They have now called on Defra to tighten up bovine TB (bTB) controls for non-bovine species in England, and argue in favour of an organised cull of badgers – the creature many farmers believe is at the centre of spreading the disease to catastrophic effect. Members of an alpaca TB support group have reported the loss of 155 animals to the disease in the first seven months of this year, in heavy contradiction to Defra's records.

The anomaly is said to have arisen because Defra's figures only show those animals which have had a postmortem examination, something not compulsory for every death.

Dianne Summers, welfare officer of the Cornwall Alpaca and Llama Group, said many herds have been infected in the West country, an area where any future badger cull is likely to be tested due to the species' density. Miss Summers said: "Defra's figures are misleading to say the least."

Alpacas can fetch anywhere from £4,000 up to £20,000 each for stud males, but if an alpaca herd is found to be infected with bTB, it can be placed under restriction preventing them from being sold or studded out.

Ian Johnson, spokesman for the NFU South West, supported calls for action and said he agreed the figures could support the argument for a badger cull.

He added: "I think it is clearly a very undesirable situation to have any kind of disease running rampant or undetected, trying to control one part of it with a Draconian regime of restricted movement and almost constant testing which leaves a legacy of stress and cost. What is the point if at the same time, we have a creature which is a carrier of the disease but not subject to any restrictions or controls? It is like trying to wipe out MRSA from hospitals by only cleaning half the wards. The bTB deaths are increasing. We are hearing about cats, pigs and even sheep getting infected, so understandably people are talking about the impact this will have on a badger cull."

A Defra spokesman said it was working to update how the figures are reported to give a clearer picture of the number of non-cattle affected by bTB. The spokesman added: "TB in animals costs Britain tens of millions of pounds a year, and the Government is committed to reducing its impact. We will be publishing a consultation on measures to tackle bovine TB later this year, and will publish plans for a full package of measures in the spring."